Chemistry Reference
In-Depth Information
For the above reasons, the energy and power densities of the EDLCs
composed of activated carbon are not as high as expected based on their
high SSA (only 4-5 Wh kg
1
in energy density and 1-2 kW kg
1
in power
density).
26
As a result, the development of new carbon-based electrode ma-
terials that can overcome the listed limits of activated carbon has been
pursued. In the next section, the new carbon nanomaterials for EDLC elec-
trodes are introduced.
d
n
3
r
4
n
g
|
4
9.3.2 Carbon-based Nanomaterials for EDLC
The high SSA and corrosion resistive properties of carbon-based nano-
materials rendered these materials most valuable for EDLC electrodes.
Furthermore, carbon is an abundant element on earth and therefore,
carbon-based nanomaterials can potentially be the most cost-effective com-
petitor among other nanomaterials. Carbon-based nanomaterials for EDLC
range from carbon nanotubes (CNTs), and graphene
1
to other graphitic or
partially graphitic carbons such as carbon fibers,
27
carbon onions,
28
carbon
nanohorns,
29
etc. In this section, we discuss the application of the most widely
studied carbon nanomaterials: carbon nanotubes and graphene.
9.3.2.1 Carbon Nanotubes
The structure of a carbon nanotube can be described as rolled graphitic
carbon sheets. The number of the sheets varies. Carbon nanotubes com-
prised of a single carbon sheet are called single-walled carbon nanotubes
(SWNT). In the same manner, nanotubes with multiple walls are called
multi-walled carbon nanotubes (MWCNT). The SSA of CNTs can reach about
1600 m
2
g
1
for SWNTs
30
and about 430 m
2
g
1
for MWNTs.
31
The graphitic
carbon atoms are strongly bound to each other and are almost inert
to foreign chemicals under the operation conditions of EDLC. The
electrical conductivity of individual CNTs is excellent, extending to the order
of 10
5
Scm
1
.
32
CNTs also have high strength (tensile strength: 10 GPa of
SWNT) and outstanding mechanical resilience. Thus electrodes of CNTs are
readily applicable to the electrodes of flexible energy storage systems.
Several large-scale manufacturing methods for CNTs can enable their
commercial use. SWNTs can be produced by a gas-phase growth mechanism,
the so-called HiPco process.
33
Catalytic chemical vapor deposition (CCVD) is
a versatile method that can produce SWNTs and MWNTs selectively by
controlling the size of the catalyst.
34
The CCVD growth can produce vertically
aligned CNTs (VACNTs) or a CNT forest. In the CNT forest, the growth of
individual nanotubes is directed vertically to the substrate. This structure is
highly promising as the electrode of supercapacitors since the electrical and
ionic transport path is straight to the current collector with minimal scat-
tering by neighboring CNTs. Owing to this unique structure, the CNT forest
can be used not only as a direct EDLC electrode but also as a mesoporous
conductive template that can be decorated with other electroactive
.
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